1. Field of the Invention
The invention relates to radiation-crosslinkable organopolysiloxane-polyurea block copolymers, to processes for preparing them, and to their use.
2. Description of the Related Art
Organopolysiloxane-polyurea block copolymers are known, and can be prepared, for example, from aminoalkyl-terminated siloxanes and diisocyanates.
The formation of hydrogen bonds between the urea groups allow such polymers to be thermoplastic elastomers; in other words, they are plastic above the softening point, while below it they have elastic properties. Hence they can be used, for example, as hotmelt adhesives. A disadvantage is that the adhesive bond is reversible by an increase in temperature beyond the softening point. Moreover, moldings or adhesive bonds produced from such polymers are subject to cold flow, because even below the softening point hydrogen bonds are able continually to separate and reattach, so that deformation and hence failure are real possibilities. Accordingly, use is limited to applications where no heightened temperatures and/or forces act on the thermoplastic elastomer.
One solution to the problem is to crosslink the individual polymer chains additionally with covalent bonds, i.e., thermally irreversible bonds. If, in production, the thermoplastic elastomers are crosslinked through the use, for example, of trifunctional units, the processing properties (e.g., melt viscosity) are adversely affected. Crosslinking after application is therefore more sensible. Light-curable elastomers are known and are described for example in DE-A 42 11 391. Photocurable liquid silicone compositions are described for example in U.S. Pat. No. 5,635,544. Light-sensitive thermoplastic compositions are described in U.S. Pat. No. 5,053,316.